Re: [PATCH bpf] bpf: Proper R0 zero-extension for BPF_CALL instructions

[Yonghong Song <yhs@meta.com>]

On 12/6/22 5:21 AM, Ilya Leoshkevich wrote:
> On Fri, 2022-12-02 at 11:36 +0100, Björn Töpel wrote:
>> From: Björn Töpel <bjorn@rivosinc.com>
>>
>> A BPF call instruction can be, correctly, marked with zext_dst set to
>> true. An example of this can be found in the BPF selftests
>> progs/bpf_cubic.c:
>>
>>    ...
>>    extern __u32 tcp_reno_undo_cwnd(struct sock *sk) __ksym;
>>
>>    __u32 BPF_STRUCT_OPS(bpf_cubic_undo_cwnd, struct sock *sk)
>>    {
>>            return tcp_reno_undo_cwnd(sk);
>>    }
>>    ...
>>
>> which compiles to:
>>    0:  r1 = *(u64 *)(r1 + 0x0)
>>    1:  call -0x1
>>    2:  exit
>>
>> The call will be marked as zext_dst set to true, and for some
>> backends
>> (bpf_jit_needs_zext() returns true) expanded to:
>>    0:  r1 = *(u64 *)(r1 + 0x0)
>>    1:  call -0x1
>>    2:  w0 = w0
>>    3:  exit
> 
> In the verifier, the marking is done by check_kfunc_call() (added in
> e6ac2450d6de), right? So the problem occurs only for kfuncs?
> 
>          /* Check return type */
>          t = btf_type_skip_modifiers(desc_btf, func_proto->type, NULL);
> 
>          ...
> 
>          if (btf_type_is_scalar(t)) {
>                  mark_reg_unknown(env, regs, BPF_REG_0);
>                  mark_btf_func_reg_size(env, BPF_REG_0, t->size);
> 
> I tried to find some official information whether the eBPF calling
> convention requires sign- or zero- extending return values and
> arguments, but unfortunately [1] doesn't mention this.
> 
> LLVM's lib/Target/BPF/BPFCallingConv.td mentions both R* and W*
> registers, but since assigning to W* leads to zero-extension, it seems
> to me that this is the case.

We actually follow the clang convention, the zero-extension is either
done in caller or callee, but not both. See 
https://reviews.llvm.org/D131598 how the convention could be changed.

The following is an example.

$ cat t.c
extern unsigned foo(void);
unsigned bar1(void) {
     return foo();
}
unsigned bar2(void) {
     if (foo()) return 10; else return 20;
}
$ clang -target bpf -mcpu=v3 -O2 -c t.c && llvm-objdump -d t.o

t.o:    file format elf64-bpf

Disassembly of section .text:

0000000000000000 <bar1>:
        0:       85 10 00 00 ff ff ff ff call -0x1
        1:       95 00 00 00 00 00 00 00 exit

0000000000000010 <bar2>:
        2:       85 10 00 00 ff ff ff ff call -0x1
        3:       bc 01 00 00 00 00 00 00 w1 = w0
        4:       b4 00 00 00 14 00 00 00 w0 = 0x14
        5:       16 01 01 00 00 00 00 00 if w1 == 0x0 goto +0x1 <LBB1_2>
        6:       b4 00 00 00 0a 00 00 00 w0 = 0xa

0000000000000038 <LBB1_2>:
        7:       95 00 00 00 00 00 00 00 exit
$

If the return value of 'foo()' is actually used in the bpf program, the
proper zero extension will be done. Otherwise, it is not done.

This is with latest llvm16. I guess we need to check llvm whether
we could enforce to add a w0 = w0 in bar1().

Otherwise, with this patch, it will add w0 = w0 in all cases which
is not necessary in most of practical cases.

> 
> If the above is correct, then shouldn't we rather use sizeof(void *) in
> the mark_btf_func_reg_size() call above?
> 
>> The opt_subreg_zext_lo32_rnd_hi32() function which is responsible for
>> the zext patching, relies on insn_def_regno() to fetch the register
>> to
>> zero-extend. However, this function does not handle call instructions
>> correctly, and opt_subreg_zext_lo32_rnd_hi32() fails the
>> verification.
>>
>> Make sure that R0 is correctly resolved for (BPF_JMP | BPF_CALL)
>> instructions.
>>
>> Fixes: 83a2881903f3 ("bpf: Account for BPF_FETCH in
>> insn_has_def32()")
>> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
>> ---
>> I'm not super happy about the additional special case -- first
>> cmpxchg, and now call. :-( A more elegant/generic solution is
>> welcome!
>> ---
>>   kernel/bpf/verifier.c | 3 +++
>>   1 file changed, 3 insertions(+)
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index 264b3dc714cc..4f9660eafc72 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -13386,6 +13386,9 @@ static int
>> opt_subreg_zext_lo32_rnd_hi32(struct bpf_verifier_env *env,
>>                  if (!bpf_jit_needs_zext() && !is_cmpxchg_insn(&insn))
>>                          continue;
>>   
>> +               if (insn.code == (BPF_JMP | BPF_CALL))
>> +                       load_reg = BPF_REG_0;
>> +
>>                  if (WARN_ON(load_reg == -1)) {
>>                          verbose(env, "verifier bug. zext_dst is set,
>> but no reg is defined\n");
>>                          return -EFAULT;
>>
>> base-commit: 01f856ae6d0ca5ad0505b79bf2d22d7ca439b2a1
> 
> [1]
> https://docs.kernel.org/bpf/instruction-set.html#registers-and-calling-convention